1. Field of the Invention
The present invention relates to a circularly-polarized-wave patch antenna. In particular, the present invention relates to a configuration of a feeder circuit thereof.
2. Description of the Related Art
In recent years, patch antennas, which are compact and thin circularly-polarized-wave antenna, have been becoming widespread. In this type of patch antenna, a main body of the antenna is formed by providing a patch electrode and a ground electrode on both principal surfaces of a dielectric substrate. In this configuration, a predetermined high-frequency signal is supplied to a feeding point of the patch electrode so as to excite two orthogonal modes whose phases are different by 90°. Accordingly, a circularly polarized radio wave is radiated.
A single-point feeding method or a two-point feeding method can be adopted in a circularly-polarized-wave patch antenna. In general, a single-point feeding method is adopted because of its simple configuration. In the circularly-polarized-wave patch antenna using a single-point feeding method, a degenerate isolation element (perturbation element), such as a notch, is loaded on the patch electrode, and only one feeding point is provided on the patch electrode. One end of a feeding pin, which extends through the dielectric substrate, is connected to the feeding point, and the other end of the feeding pin is connected to a feeder line, such as a coaxial cable. In the patch antenna of a single-point feeding type configured in the above-described manner, by adequately adjusting an area ratio of the patch electrode to the degenerate isolation element and the position of the feeding point, a phase difference of 90° can be generated between two orthogonal modes, having an equal amplitude and a different resonance length. Accordingly, the patch antenna can be operated as a circularly-polarized-wave antenna.
However, in the circularly-polarized-wave patch antenna using the single-point feeding method, a band of resonance-frequency for generating a phase difference of 90° between the two orthogonal modes is narrow. Therefore, a bandwidth in which a satisfactory axial ratio characteristic required for the circularly-polarized-wave antenna can be obtained, that is, a bandwidth in which the axial ratio of an elliptically polarized wave is under a permissible value, is quite narrow. Accordingly, a favorable axial ratio characteristic cannot be obtained in a wide band.
On the other hand, in a patch antenna using the two-point feeding method, a patch electrode is circular or square-shaped and a degenerate isolation element is not loaded thereon. Two signals whose phases are different by 90° are supplied to two feeding points provided on the patch electrode. A 90°-phase-difference circuit is provided between the input port of a feeder circuit and the patch antenna. With this configuration, a phase of one signal supplied to one of the feeding points of the patch antenna is always delayed by 90° with respect to a phase of another signal supplied to the other feeding point. Accordingly, the two orthogonal modes of the patch electrode are excited with a phase difference of 90°, and thus the patch antenna can be operated as a circularly-polarized-wave antenna. In the patch antenna using the two-point feeding method, signals whose phases are different from each other by 90° are supplied to the two feeding points so as to excite the two orthogonal modes. As a result, a favorable axial ratio characteristic can be obtained over a wide frequency band.
As described above, a favorable axial ratio characteristic can be obtained in a wide band by adopting a circularly-polarized-wave patch antenna including two feeding points. However, in a known patch antenna of a two-point feeding type, it is not easy to supply electric power to the two feeding points of the patch electrode over a wide frequency band without reflection. Further, since reflection of signal waves is more likely to increase due to the limited frequency band of the patch antenna itself, a favorable reflection characteristic cannot be obtained in a wide band. This is because isolation of a pair of transmission lines of the 90°-phase-difference circuit connected to the patch electrode is difficult to ensure.